1. Field of the Invention
The present invention relates to a semiconductor laser device and a method for fabricating the same, and a strained quantum well crystal and a method for fabricating the same.
2. Description of the Related Art
Referring to FIGS. 15A and 15B, a conventional strained quantum well type semiconductor laser device will be described. Such a strained quantum well type semiconductor laser device is disclosed, for example, in Temkin et al., J. Cryst. Growth, 93, 353, (1988).
As shown in FIG. 15B, the semiconductor laser device includes: an InP single crystalline substrate 1; a stripe-shaped multi-layered structure 10 provided on the substrate 1; and Fe-InP current blocking layers 7 for covering both sides of the multi-layered structure. An n-type electrode 9 is provided on the bottom surface of the semiconductor laser device and a p-type electrode 8 is provided on the upper surface of the semiconductor laser device. A dielectric film containing SiO.sub.2 and/or Si is provided on an end surface of the semiconductor laser device.
The stripe-shaped multi-layered structure 10 includes: a first optical waveguide layer 2; a multi-quantum well portion 5 consisting of a strained quantum well layer 3 and a barrier layer 4; and a second optical waveguide layer 6. The energy band gaps in this portion are shown in FIG. 15A.
When a current is supplied from the p-type electrode 8 to the n-type electrode 9, holes and electrons are confined in the multi-quantum well portion 5, so that the holes and the electrons are recombined so as to generate an optical emission. As a result, a laser oscillation is generated in the semiconductor laser device. In this conventional example, the strained quantum well layer 3 is composed of mixed crystals having a composition of In.sub.0.7 Ga.sub.0.3 As, and is formed so as to have a compression strain of 1%. By introducing the compression strain into the well layer 3, the band structure of the holes can be similar to the band structure of the electrons. As a result, even if a small amount of carriers are injected, the laser oscillation is generated in the semiconductor laser device.
However, in such a semiconductor laser device, as the temperature of the semiconductor laser device increases, the wavelength of the laser oscillation is shifted to the longer wavelength side. Therefore, such a semiconductor laser device has a problem in that the oscillation wavelength thereof is variable in accordance with the variation of the temperature of the semiconductor laser device.
In addition, if the amount of the current to be injected into the semiconductor laser device is increased, the temperature of the semiconductor laser device is also increased; consequently the wavelength of the laser oscillation is considerably varied depending upon the amount of the injected current.
On the other hand, in the case where such a strained quantum well structure is applied to a distributed feedback type laser device (hereinafter, simply referred to as a "DFB type laser device"), the variation in the temperature of the semiconductor laser device makes it difficult to realize a single mode oscillation. In other words, a side mode is generated in the semiconductor laser device, and the difference between the peak at the oscillation wavelength and the peak of the side mode cannot be set to be large. Such a phenomenon is remarkable in a strained quantum well laser device, in particular, because the introduction of the strain to the laser device reduces the threshold carrier density and causes an abrupt gain for the laser device.
FIG. 12A shows the profiles of the gain spectra varying depending upon the temperature in a semiconductor laser device having a non-strained multiple-quantum well (MQW) structure, while FIG. 12B shows the profiles of the gain spectra varying depending upon the temperature in a semiconductor laser device having a strained MQW structure.
As shown in FIG. 12A, since the gain is not so large in the laser device having the non-strained MQW structure, substantially flat gain spectrum can be obtained in a wide wavelength region. The wavelength at the peak of this gain spectrum is shifted to a longer wavelength side as the temperature (T) of the laser device is set to be higher (high T). However, since the gain spectrum is flat in the laser device having the non-strained MQW structure, the difference between the peak at the oscillation wavelength and the peak of the side mode can be set to be large. In other words, it is easier to realize the single mode oscillation for the laser device. The reason is as follows. Even when the variation of the temperature makes longer the distance between the wavelength at the peak of the gain spectrum and the DFB oscillation wavelength, the gain can still be obtained at the DFB oscillation wavelength, so that the laser device can preferentially oscillate at the DFB oscillation wavelength.
On the other hand, in the case where the laser device has a strained MQW structure, the introduction of the strain locally increases the gain in a particular wavelength region, as shown in FIG. 12B. If the ambient temperature is low (low T), the DFB oscillation can still be generated even when the wavelength at the peak of the gain spectrum deviates from the DFB oscillation wavelength to a certain degree. As a result, the difference between the peak at the oscillation wavelength and the peak of the side mode can be set to be large.
However, if the temperature becomes high (high T), the wavelength at the peak of the gain spectrum is shifted to the longer wavelength side in the same way as in the laser device having the non-strained MQW structure, so that substantially no gain can be obtained at the DFB oscillation wavelength. As a result, not only the decrease in the gain at the DFB oscillation wavelength but also the laser oscillation at the wavelength at the peak of the gain spectrum are caused. Consequently, the difference between the peak at the oscillation wavelength and the peak of the side mode cannot be set to be large and it becomes difficult to realize a single mode oscillation for the laser device. That is to say, since the strained quantum well laser device, in particular, has an abrupt gain profile, even when the distance between the DFB oscillation wavelength and the wavelength at the peak of the gain spectrum is short, i.e., the amount of the detuning to be performed is small, the difference between the peak at the oscillation wavelength and the peak of the side mode becomes disadvantageously small and it becomes difficult to realize the single mode oscillation. Therefore, even if a smaller amount of the detuning is required for a strained quantum well laser device as compared with a non-strained quantum well laser device, the single mode oscillation cannot be realized easily. Accordingly, a semiconductor laser device with a small degree of the temperature dependence of a gain wavelength is desired.